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Pyroelectric response

Posted Jan 8, 2017, 6:06 p.m. EST 2 Replies

Alan Davidson
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I have a piezoelectric buzzer (material unknown but presumed to be PZT) I am testing as a possible low frequency pressure sensor. I have the buzzer connected to a charge amplifier I built to have a very large time constant. While this allows me to measure very low frequency pressure signals, the long time constant means that the pyroelectric effect is significant allowing use as a temperature sensor. I am however puzzled that an increase in temperature produces a signal with the opposite polarity of that produced by a compressive force. Since the d33 of PZT (and other piezo-ceramics) is positive, I would have expected both an increase in temperature and a compressive force to produce an output signal of the same polarity since both compressive forces and increases in temperature produce a reduction in the remanent polarisation in the poled piezoelectric. If anyone can help explain this anomaly I would be eternally grateful!
2 Replies Last Post Jan 9, 2017, 4:49 p.m. EST
Dave Greve Certified Consultant
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Posted: 7 years ago Jan 9, 2017, 3:02 p.m. EST
It's hard to say without knowing more about the complete physical structure.

However- if the piezo is bonded to a metal- when the temperature increases the metal expands and will cause a stress in the piezo and hence a polarization. You would have to know thermal expansion coefficients for both materials to know how the signs work out.

DWGreve
DWGreve Consulting
It's hard to say without knowing more about the complete physical structure. However- if the piezo is bonded to a metal- when the temperature increases the metal expands and will cause a stress in the piezo and hence a polarization. You would have to know thermal expansion coefficients for both materials to know how the signs work out. DWGreve DWGreve Consulting
Alan Davidson
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Posted: 7 years ago Jan 9, 2017, 4:49 p.m. EST
Many thanks for your reply. The buzzer piezoelectric is bonded to a brass substrate and has a silver top electrode. The coefficient of thermal expansion for brass and silver is much greater than that of PZT and brass is about 60% stiffer than PZT so this should result in the PZT being 'stretched' along it's base resulting in an enhanced reduction in the spontaneous polarisation of the PZT due to the positive strain induced - even more than the result of the thermal expansion of PZT alone. After some thought I am thinking that the increase in polarisation I see is due to the heat source (a Peltier tile) heating the surface of the PZT in the centre only resulting in an expansion only in the 3-direction (perpendicular to the surface of the PZT) close to the surface which in the absence of heating within the PZT (that would cause a reduction in the spontaneous polarisation) would result in the polarisation increase. I'll try the experiment again and give the PZT longer to reach an even temperature. I doubt I will see any difference since the effect I've see is large - even placing a finger close to the silver electrode produces a considerable response! Thank you again for your thoughts.

Best regards,

Alan
Many thanks for your reply. The buzzer piezoelectric is bonded to a brass substrate and has a silver top electrode. The coefficient of thermal expansion for brass and silver is much greater than that of PZT and brass is about 60% stiffer than PZT so this should result in the PZT being 'stretched' along it's base resulting in an enhanced reduction in the spontaneous polarisation of the PZT due to the positive strain induced - even more than the result of the thermal expansion of PZT alone. After some thought I am thinking that the increase in polarisation I see is due to the heat source (a Peltier tile) heating the surface of the PZT in the centre only resulting in an expansion only in the 3-direction (perpendicular to the surface of the PZT) close to the surface which in the absence of heating within the PZT (that would cause a reduction in the spontaneous polarisation) would result in the polarisation increase. I'll try the experiment again and give the PZT longer to reach an even temperature. I doubt I will see any difference since the effect I've see is large - even placing a finger close to the silver electrode produces a considerable response! Thank you again for your thoughts. Best regards, Alan

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